1. Field of the Invention
The present disclosure relates to a power optimization device and method for energy harvesting apparatus, more particularly to a power optimization device capable of adjusting operating frequency of a charge pump circuit to track a maximum power point for optimizing the power outputted from the energy harvesting apparatus.
2. Description of the Related Art
Energy harvesting technology, which is used to harvest energy from the nature (such as light, heat, electromagnetic energy or vibration), has been applied in many fields for a long time. The way of harvesting such energy is similar to that of green power, but the difference between them is that the energy harvesting technology is used to harvest energy from a very weak energy source.
With progress of the integrated circuit and network technologies, many companies pay more attention in applications of wearable devices and internet of thing (IoT), and the energy harvesting technology is closely related to these two applications. For example, in the field of IoT, many wireless sensor networks (WSN) are required to detect and transmit data, but it also takes a lot of time and labor costs to connect a large number of WSNs with wired electric power grids or replace their batteries. The energy harvesting technology can enable the WSNs to obtain energy from surroundings, so as to reduce dependence on wired electric power grid or the battery. In addition, subject to a relatively small volume, the wearable product is usually provided with a small-size battery and has a limited operating time due to the limited power supplied by the small-size battery. If the energy harvesting technology can be applied in the wearable product, the wearable product can obtain energy from surroundings rather than from the battery, so it is beneficial to save battery life and reduce times of replacing or charging of the battery of the wearable product.
In order to obtain the energy from surroundings, a transducer is required to convert the energy from the surroundings (such as light energy or heat) into voltage or current which is acceptable for IC. A solar cell converting light energy into electrical energy is taken as an example for illustration below. Please refer to FIG. 1 which shows output current versus output voltage curves and output power versus output voltage curves of the solar cell. As shown in FIG. 1, a conversion curve of the solar cell is nonlinear and different from an ideal voltage or current source, and has a maximum value called as a maximum power point (MPP). In order to obtain the maximum power, the power converter connected to the output of the solar cell preferably keeps the working point in the maximum power point. Please refer to FIG. 1B which shows output current versus output voltage curves and output power versus output voltage curves of a common piezoelectric converter. The piezoelectric converter has I-V characteristic different from the solar cell, but also has a maximum power point.
After the transducer converts the energy from the nature into the electrical energy, the power converter converts the output of the transducer into the voltage and current which is acceptable for a post-stage system. Generally speaking, the power outputted from the transducer is very weak, so the power converter is required to have very high power conversion efficiency. A DC/DC switching converter, which has very high conversion efficiency, is widely applied in various applications. For example, a buck converter can be applied in fields of the solar cell and vibration energy harvesting, and a boost converter can be applied in fields of the solar cell and heat harvesting. Although having high conversion efficiency, the conventional DC/DC switching converter needs an inductor as an energy storage element. However, the inductor usually has a large volume and is disadvantageous to the wearable product.